Electric motors typically include a rotor mounted on a shaft inside a stator. The rotor can have conducting elements, sometimes called rotor bar, placed along its periphery. The rotor bars can be parallel with the shaft or can be skewed in relation to the shaft. At each end of the rotor an end ring can be connected to the respective ends of the rotor bars, for example as in done in so-called squirrel-cage rotors. The rotor bars and end rings are made from some suitable material, such as copper.
Because rotors will revolve in normal operation, it is important that they be properly balanced before the electric motor can be used. Balancing is sometimes preceded by spinning the rotor at relatively high speed. This puts a high load on the rotor components. For example, spinning at 16,000 rpm can subject the copper rotor bars to a load on the order of 10 kN. Likewise, the end rings, which can also be made of copper, can be subjected to a high centripetal shear, on the order of 181 MPa.
Loads of these magnitudes can affect the rotor balance. For example, the end ring material (e.g., copper) can expand outward due to the load impacted by the spinning. That is, the outer diameter of the end ring can increase due to the intense rotation, making the end ring larger than initially. As another example, the end ring can shift so that it is no longer coaxial with the rotor shaft, causing the rotor to become unbalanced.
It is sometimes attempted to counteract these and other sources of imbalance by selectively removing small amounts of material from the end rings. After such removal, the rotor is again spun at high speed to determine whether the adjustment was sufficient. However, such additional rotation processes can again introduce some amount of imbalance, as discussed above.